Yuguang Chen

Correlation effects on the dynamics of bipolarons in nondegenerate conjugated polymers

By employing an adaptive time-dependent density-matrix-renormalization-group method, we investigate the dynamics of a charged bipolaron in the presence of both electron-phonon and electron-electron interactions. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via a Hubbard Hamiltonian, a Brazovskii-Kirova symmetry-breaking term, and an external electric field. Our results show that the velocity of the bipolaron increases first and then decreases with the increasing of the on-site Coulomb interaction, U. Furthermore, the dependence of the bipolaron velocity, bipolaron effective mass, and bipolaron stability on the lattice structures is discussed.

Spin-flip processes of polarons by magnetic impurities in conjugated polymers

By employing an adaptive time-dependent density-matrix-renormalization-group method, the spin-flip process of polarons is investigated in a polymer chain with magnetic impurities. Being driven by an external electric field, a polaron carrying both spin 1/2 and charge +/-e moves at a constant speed in the polymer chain. When the polaron passes through a specific site, which couples to a magnetic impurity via spin-exchange interaction, a spin-flip process is observed if its spin is antiparallel to the impurity spin. Our results show that the spin-flip probability is enhanced by the on-site Coulomb interaction and increases with increase in the spin-exchange integral. Additionally, some possible applications of the spin-state swap between the polaron and the impurity are discussed.

Spin filtering in a nonuniform quantum wire with Rashba spin–orbit interaction

Abstract We investigate theoretically the spin-polarized electron transport for a wide-narrow-wide (WNW) quantum wire under the modulation of Rashba spin–orbit interaction (SOI). The influence of both the structure of the quantum wire and the interference between different pairs of subbands on the spin-polarized electron transport is taken into account simultaneously via the spin-resolved lattice Green function method. It is found that a very large vertical spin-polarized current can be generated by the SOI-induced effective magnetic field at the structure-induced Fano resonance even in the presence of strong disorder. Furthermore, the magnitude of the spin polarization can be tuned by the Rashba SOI strength and structural parameters. Those results may provide an effective way to design a spin filter device without containing any magnetic materials or applying a magnetic field.

Concurrence and fidelity of a Bose-Fermi mixture in a one-dimensional optical lattice

We study the ground-state fidelity and entanglement of a Bose-Fermi mixture loaded in a one-dimensional optical lattice. It is found that the fidelity is able to signal quantum phase transitions between the Luttinger liquid phase, the density-wave phase, and the phase separation state of the system, and the concurrence, as a measure of the entanglement, can be used to signal the transition between the density-wave phase and the Ising phase.We study the ground-state phase diagram of a Bose-Fermi mixt ure loaded in a one-dimensional optical lattice by computing the ground-state fidelity and quantum entangle ment. We find that the fidelity is able to signal quantum phase transitions between the Luttinger liquid pha se, the density-wave phase, and the phase separation state of the system; and the concurrence can be used to signal the transition between the density-wave phase and the Ising phase.

Bosonization study of quantum phase transitions in the one-dimensional asymmetric Hubbard model

The quantum phase transitions in the one-dimensional asymmetric Hubbard model are investigated with the bosonization approach. The conditions for the phase transition from density wave to phase separation, the correlation functions, and their exponents are obtained analytically. Our results show that the difference between the hopping integrals for up- and down-spin electrons is crucial for the occurrence of the phase separation. When the difference is large enough, the phase separation will appear even if the on-site interaction is small.

Low energy excitation of the one-dimensional Kondo-necklace model

Abstract The one-dimensional Kondo-necklace model is investigated on the basis of bosonization. We prove that the model belongs to the same universality class as the one-dimensional half-filling Kondo lattice model at operator level, furthermore, the excitation and the gap formation are derived. We suggest that the spin gap of this model always exists for all finite J .

Critical behaviour of organic ferromagnets

The phase diagram of the quasi-one-dimensional model which is most relevant to organic ferromagnets of a certain kind is considered. In this model, strong electron-phonon interaction is considered adiabatically and the coupling between local spins and π-electrons is treated within a mean-field theory. A full phase diagram is given. The results show that there are two new kinds of phase which were not revealed in previous treatments. Our results may be helpful for the synthesis of organic ferromagnets.

Spin polarization direction switch based on an asymmetrical quantum wire

Abstract A scheme for a spin polarization direction switch is investigated by studying the spin-dependent electron transport of an asymmetrical quantum wire with Rashba spin–orbit coupling. It is found that the spin polarization direction can be switched by changing the direction of the external current. The physical mechanism of this device arises from the fact that the symmetries in the longitudinal and transverse directions are all broken but C 2 -rotation and time-reversal symmetries are still preserved. Further studies show that the spin polarization is robust against disorder, displaying the feasibility of the proposed structure for a potential application.

Local spin polarisation of electrons in Rashba semiconductor nanowires: effects of the bound state

Abstract The local spin polarisation (LSP) of electrons in two typical semiconductor nanowires under the modulation of Rashba spin-orbit interaction (SOI) is investigated theoretically. The influence of both the SOI- and structure-induced bound states on the LSP is taken into account via the spin-resolved lattice Green function method. It is discovered that high spin-density islands with alternative signs of polarisation are formed inside the nanowires due to the interaction between the bound states and the Rashba effective magnetic field. Further study shows that the spin-density islands caused by the structure-induced bound state exhibit a strong robustness against disorder. These findings may provide an efficient way to create local magnetic moments and store information in semiconductors.

Correlation effects on the scattering of soliton pairs in conjugated polymers

By employing an adaptive time-dependent density matrix renormalization group (t-DMRG) method, we investigate the electron-electron correlation effects on the inelastic-scattering processes of oppositely charged-soliton anti-soliton pairs in conjugated polymers under the influence of an external electric field. The simulations are performed based on the Su-Schrieffer-Heeger (SSH) model modified to include electron-electron interactions and an external electric field. The percent yield of product is numerically calculated by a projection method. Our results show that the on-site Coulomb interaction is of fundamental importance and favors the recombination between the pairs of soliton. Additionally, the effects of external electric field and nearest-neighbor electron-electron interaction on the collision processes are also discussed.